Asynchronous machines are widely used for various drive purposes. When an asynchronous machine is being started, it is of interest to limit the starting currents and the starting torque of the asynchronous machine, and to increase them slowly as the rotation speed of the asynchronous machine rises. Soft starting such as this is of interest for various technical applications. Soft starting can be achieved by controlling the electrical power which is supplied to the asynchronous machine. By way of example, the electrical power can be regulated by phase-gating control.
In particular asynchronous machines may be used to drive tools or machines with a high mass moment of inertia. For example, machines such as these may be planing machines, milling machines, circular saws etc., in the carpentry industry. For safety reasons, particularly for machines such as this, it is necessary for the run down after the machine has been switched off to be as short as possible. Typically, run down times of between 10 seconds and 20 seconds are required. The run down time of the asynchronous machine should at least be no longer than the run up time.
In order to brake an asynchronous machine, it is known for a DC voltage or a rectified AC voltage to be applied to two phases of the machine, in order to achieve a braking effect in this way. However, this so-called DC braking has various technical disadvantages. If the DC braking process for a machine is provided by pure direct current, then a direct-current source is required for this purpose. This is technically complex to implement. If the DC braking process is provided by a rectified AC voltage within phase-gating control, by in each case using only one half-cycle of the rectified voltage for DC braking (this corresponds in principle to half-wave rectification), then only a small braking torque can be produced because of the relatively short times during which current flows. Furthermore, the asymmetric current profiles cause losses within the asynchronous machine. For this reason, the required short run down times for machines or tools with very high mass moments of inertia frequently cannot be achieved simply by DC braking.
In order to achieve the required braking times, it is often essential to carry out a braking process using a frequency converter. However, frequency converters occupy valuable physical space, and are also expensive to obtain.